As is known in the art, there are several types of active devices used at microwave and millimeter wave frequencies to provide amplification of radio frequency signals. In general, one of the more common semiconductor devices used at these frequencies are field effect transistors, in particularly metal semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs). Each of these transistors is provided from Group III–V materials such as gallium arsenide. What distinguishes a HEMT from a MESFET is that in a HEMT charge is transferred from a doped charge donor layer to an undoped channel layer whereas in a MESFET the charge layer and the channel layer are the same layer. Due to the presence of an undoped channel layer in a HEMT, charge transport properties of the undoped channel layer are better than those of the doped channel layer of a MESFET type structure. Accordingly, HEMTs provide higher frequency operation than MESFETs.
In a HEMT, the charge donor layer is generally a wide bandgap material, such as aluminum gallium arsenide whereas the channel layer is a lower bandgap material, such as gallium arsenide or indium gallium arsenide. It is to be noted that bandgap refers to the potential gap between valance and conduction bands of the semiconductor materials.
In general, there are two types of HEMT structures. One type is simply referred to as a HEMT, whereas the other type is referred to as a pseudomorphic HEMT, or pHEMT. The difference between the HEMT and the pseudomorphic HEMT is that in the pseudomorphic HEMT one or more of the layers incorporated into the HEMT structure is comprised of a material having a lattice constant which differs significantly from the lattice constants of the other materials of the device. Thus, due to resulting lattice constant mismatch, the crystal structure of the material providing the channel layer is strained.
As noted above, in a HEMT structure, charge is transferred from the donor layer to an undoped channel layer. For Group III–V materials, a doped charge donor layer is comprised of a wide bandgap material, such as gallium aluminum arsenide, whereas the channel layer is typically comprised of a material having better electron transport properties. Typically, a lower bandgap material, such as gallium arsenide is used. In a pseudomorphic HEMT, the undoped gallium arsenide channel layer is replaced by a channel layer comprised of a lower bandgap material, such as indium gallium arsenide. In either event, however, each of the HEMT and pseudomorphic HEMT structures are used to provide amplification of high frequency microwave and millimeter wave signals.
For low noise and high frequency applications of high electron mobility transistors, it is important to have a narrow recess disposed through the contact layers of the device and over the charge donor layer. That is, the recess opening is preferably only slightly longer than the gate length of the gate electrode disposed within the recess. This arrangement has provided HEMTs and pseudomorphic HEMTs that have relatively high frequency operating characteristics and relatively low noise figures. For power applications in MESFETs, it is generally known that a recessed opening larger than the gate is necessary to provide the MESFET having relatively high gate to drain breakdown voltage characteristics.
Returning to a HEMT, on the etched gallium aluminum arsenide surface which is generally the upper surface in most HEMT structures, there exists a large number of surface states. Such surface states also exist on the GaAs surface. Some authors have estimated the surface states to be as many as 1014 cm−2. These states most likely arise from gallium and aluminum oxides. It has been suggested that these states once occupied, increase the gate to drain breakdown voltage characteristic by capturing electrons and thus decreasing the electric field concentrated at the gate metal edge on the drain side of the transistor.
The breakdown voltage characteristics of high electron mobility transistors has limited their use to relatively low power, low noise applications. This follows since the output impedance of a HEMT is generally related to the drain bias level. Low breakdown voltage characteristics limits the operating drain voltage of a HEMT. For a given DC power level, it is general advantageous to bias a HEMT for high power applications at relatively high drain voltages and low drain current rather than vice versa. Biased at high drain voltage provides a higher output impedance for the HEMT and therefore a more easy impedance match to a 50 ohm system characteristic impedance which is generally encountered in most applications. In particular, this match is more easily made over broad ranges of operating frequencies. Further, to provide high levels of RF voltage gain from such a device it is generally necessary to operate the device at a relatively high drain voltage DC bias. However, as indicated above, although it would be desirable to bias HEMTs at higher breakdown voltage, such is generally not possible since the HEMTs have relatively low breakdown voltage characteristics.
Therefore, high electron mobility transistors are used in relatively low power, low noise, applications, because the known high electron mobility transistors generally have relatively low gate to drain reverse breakdown voltage characteristics. This situation is undesirable since the high frequency characteristics of HEMTs and the relatively high gain of HEMTs in comparison to MESFETs, would otherwise be useful for higher power applications.
As is also known in the art, in some applications it is desirable to have some transistors on a substrate operate with, for example, radio frequency signals, while other transistors of the same substrate provide high-speed digital logic functions. One type of high-speed logic using HEMTs is a Direct Coupled FET Logic (DCFL) circuit. In such application, a plurality of FETs has different threshold values for example, a DFET (Depletion type FET) and an EFET (Enhancement type FET) formed on the same semiconductor substrate.
More particularly, Enhancement/Depletion (E/D) pHEMT technology appears to be the next step in the evolution of multifunction MMIC technology for use on such things as electronically steered arrays for radar, communication systems. E/D pHEMT offers a significant improvement in performance, size and cost over conventional GaAs based multifunction MMIC technology. One drawback implementing Direct Coupled FET Logic (DCFL) with GaAs based pHEMT is the low voltage swing (approximately 600 mv). This low voltage swing is due to the intrinsic material property of AlGaAs with 23% aluminum mole fraction. Also, in order to implement good performance DCFL, another important factor is to have uniform threshold voltage across the wafer.